When oil is produced from unconsolidated formations, migration into the wellbore of loose sand and erosion from sandstone is a constant problem. This migration of sand may eventually clog flow passages in the production system of the well and can erode downhole and surface equipment. In some instances, the clogging of the production system may lead to complete cessation of flow, or "killing" of the well.
Sand migration is typically controlled by placement of a gravel pack around a slotted or screened production liner. The "gravel" used in such a gravel pack is actually a sand of a size which is large enough to be kept out of the production liner or screen, but small enough to prevent migration of formation sand past the gravel pack and into the production facilities.
A gravel pack is generally placed by first inserting the slotted or screened liner in the portion of the wellbore to be packed. A "crossover" tool is positioned on top of the liner. The crossover tool and liner are suspended from a drill pipe. The crossover tool allows a slurry of sand in a fluid carrier to be pumped down a drill string to the crossover tool, and then be routed through the crossover tool to the volume surrounding the slotted or screened liner. The sand is deposited within this volume and the perforations. In a circulating gravel pack, some of the liquid carrier of the gravel pack slurry enters the slotted or screened liner where it can communicate through on to the annulus surrounding the drill string. The rest of the fluid leaks out to the formation through the perforations. A packer is placed above the liner in the annulus surrounding the crossover tool to prevent the slurry around the liner from bypassing the liner and communicating directly to the annulus around the drill pipe. After the sand is deposited around the liner and in the perforations, the crossover tool is disconnected from the liner and lifted out of the wellbore. A production tubing is then lowered into the wellbore and connected to the liner.
The gravel pack slurry must be sufficiently viscous to suspend the sand, and must be sufficiently shear-thinning to be pumpable. This viscosity characteristic is often provided by a polysaccharide. Further, after the sand is placed in the wellbore around the liner, it would be desirable to have the sand settle from the slurry suspension as quickly as possible. Lifting of the crossover tool from the liner before the slurry settles into a dense sand bed can disturb the gravel pack. Therefore, the gravel pack tool is typically left stationary for several hours after the gravel is placed. This has been found to be necessary with prior art gravel pack slurries, but is very expensive due to the cost of the rig time required.
It would be preferable to have a gravel pack slurry which would rapidly become less viscous after being placed in the wellbore. This would allow timely removal of the crossover tool without disturbing the freshly placed gravel pack.
Breakers have been included in gravel pack slurries to decrease viscosity after placement of the gravel pack. U.S. Pat. No. 4,662,447 discloses a method wherein acid is mixed with a gravel slurry at the crossover tool. This method comprises spotting an acid solution in the annulus around the drill string, above a crossover tool. A pad of clean fluid follows the acid, followed by a gravel pack slurry. The crossover tool is provided with a mixing valve which permits annulus fluids to flow into the crossover tool and mix with fluids being pumped down the drill string. The mixing valve is provided with a check valve, allowing fluids to pass from the annulus into the crossover tool when the pressure in the annulus exceeds that in the crossover tool contents. The acid is mixed with the gravel packing slurry by applying pressure to the annulus as the gravel packing fluid is being forced into the wellbore through the crossover tool. With this method, the annulus cannot be used to return slurry carrier fluids. The slurry carrier fluids are disbursed into the formation. This method provides some control over the timing of the mixing of acid and slurry. But the rate of the mixing is difficult or impossible to control. The differential pressures between the annulus and the drill string content at the crossover tools vary depending upon many things that are not easily determined or controlled. It would also be preferable to utilize the annulus around the drill string to return slurry carrier fluids rather than to push these fluids into the formation.
A particularly useful polysaccharide for imparting viscosity to gravel pack slurries is succinoglycan. This polymer and its properties are disclosed in, for example, ACS Symposium Series 396 "Oil Field Chemistry--Enhanced Recovery and Production Simulation" by A. J. Clarke-Sturman, et al. Chapter 8, pp 157-168; "Succinoglycan: A New Biopolymer for the Oil Field" by A. J. Clarke-Sturman, et al. from Symposium on Advanced Oil Field Chemistry presented before the Division of Petroleum Chemistry, Inc. ACS, Toronto Meeting, Jun. 5-11, 1988; and European Patent Application 0,040,445. These polymers impart a high and relatively temperature insensitive viscosity to an aqueous composition below the polymers' transition temperature. As the transition temperature is exceeded, the viscosity imparted to an aqueous mixture by these polymers decreases markedly. The transition temperatures may be adjusted to a certain extent by addition of various modifiers, as disclosed in Great Britain Patent Application No. 902 6596.8 and the A. J. Clark-Sturman articles cited above.
These biopolymers also become much more susceptible to degradation by acid breakers at temperatures near the transition temperature. U.S. Pat. No. 4,754,809 discloses a method which utilizes the rapid acid degradation above the polymers transition temperature and the adjustment of the transition temperature to a temperature near the reservoir temperature. While the slurry is in the wellbore, it heats to the reservoir temperature and a rapid degradation then results. Such a method is disclosed as being useful for well completion and workovers along with stimulation, acidizing and enhanced oil recovery. But adjustment of the transition temperature is expensive and subject to change due to dilution by reservoir fluids. Adjustment of the transition temperature to the formation temperature and breaking of the carrier gel as it heats to the formation temperature results in a very unpredictable reduction of gel viscosity. The temperatures within the borehole may not relate to the formation temperature upon placement of the gravel pack, and may equilibrate to the formation temperature at widely varying rates due to cooling from fluids lost to the formation prior to gravel packing. Relying on the gel reaching a formation temperature therefore results in considerable uncertainty as to the time required for the gel to be broken.
It is, therefore, an object of this invention to provide a method to place a gravel pack in which sanding out of the solids does not occur prematurely, but yet results in rapid breaking of the polymer upon placement of the gravel pack within the wellbore. It is a further object to provide such a method which incorporates materials which will not be damaging to the formation. It is another object of the present invention to provide such a method wherein the gravel packing tool may be removed from the vicinity of the wellbore which is gravel packed within three hours of the placement of the gravel pack without disturbing the gravel pack. It is another object of the present invention to provide such a method which can be adjusted for accommodation of a variety of formation temperatures.